Plasma arc torch systems are widely used in the cutting of materials. A plasma arc torch system generally includes a torch, an electrode mounted within the torch, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. nitrogen or argon) or reactive (e.g. oxygen or air).
Plasma arc torch systems use a starting circuit to initiate an arc. Referring to FIG. 1, a pilot arc relay is initially closed. A power source is turned on and the initial path for DC current from the source is between the electrode (cathode) and the nozzle (anode). A plasma gas from a gas supply flows between the electrode and the nozzle. There are several known ways to initiate the arc. The system can include electrical starting circuitry that generates a high frequency, high voltage spark discharge. After the initial high voltage spark discharge, the power supply continues to supply power to the pilot arc. The system can also include starting circuitry that implements "contact starting," which is described in commonly owned U.S. Pat. No. 4,791,268. In a contact starting system, the arc is initiated with the electrode and nozzle in physical contact. The power supply provides power and the electrode and nozzle are separated, drawing out the pilot arc. Using either approach, a pilot arc is formed between the electrode and the nozzle.
The pilot arc ionizes the plasma gas passing between the nozzle and the electrode and exiting the nozzle exit orifice. The torch is positioned adjacent the workpiece such that the arc contacts the workpiece and current begins to flow through the workpiece. The workpiece current is sensed by a current sensor, which is connected to a control circuit. The control circuit opens the pilot arc relay causing the arc current to flow through the electrode and the workpiece. Additionally, the control circuit increases the current level from the power supply to a higher level for the cutting operation. The torch is operated in this transferred plasma arc mode, characterized by the conductive flow of ionized gas from the electrode to the workpiece, during cutting of the workpiece. The cutting operation continues in the transferred arc mode until the arc is extinguished by turning off the DC power supply or by cutting to the end of the workpiece and drawing out the arc beyond the voltage capability of the power supply.
Problems arise in maintaining a continuous arc with a plasma arc torch system having the foregoing starting circuitry when the distance between the torch and the workpiece (i.e:, the standoff distance) becomes too large. This occurs when cutting a discontinuous (or grated) workpiece, when the torch is moved from one workpiece to another, or when the torch is disposed over open space. More specifically, the arc extinguishes when the torch is not adjacent the workpiece material, but is disposed over a discontinuity (or grating) or sufficient open space. Once the arc is extinguished, the starting circuitry resequences through the ignition process taking several seconds to restart the torch. Sensing the loss of workpiece current and reconnecting the nozzle does not solve the problem because the arc extinguishes before the loss of workpiece current can sensed.
One solution to this problem is described in commonly owned U.S. Pat. No. 4,996,407. Referring to FIG. 2, a constant pilot arc is maintained by a pilot arc transfer controller which includes additional electrical control circuitry incorporated into the starting circuitry. The transfer controller senses the voltage between the nozzle and the workpiece. When that voltage approaches some fixed voltage which is less than the maximum output voltage available from the power supply, the controller connects the nozzle to the power supply causing the arc to switch from the workpiece to the nozzle.
This solution works quite well, but is expensive due to the required additional voltage sensing, control and power switching circuitry. The additional circuitry is either isolated using additional, expensive analog isolation amplifiers or is floated at the high output voltage levels. Also, this solution requires the nozzle to be connected above a certain threshold voltage determined under worst case low AC input line voltage conditions. When the AC input voltage is nominal or high, controller switches the arc to the nozzle at a voltage which is lower than that the voltage required to maintain the arc. This premature switching prevents nominal or excess cutting capacity from being utilized.
Referring to FIG. 3, another solution to this problem involves a continuous connection to the nozzle through a power resistor. Current flow through the nozzle and resistor develops a voltage drop with a large resulting power loss. When the workpiece is positioned adjacent the torch, the current prefers to flow through the workpiece path due to the lower voltage drop. This approach is not preferred in hand-held torch systems because of the large power losses.